Add support for __wchar_t in -fms-extensions mode.

MSVC provides __wchar_t. This is the same as the built-in wchar_t type
from C++, but it is also available with -fno-wchar and in C.

The commit changes ASTContext to have two different types for this:

  - WCharTy is the built-in type used for wchar_t in C++ and __wchar_t.

  - WideCharTy is the type of a wide character literal. In C++ this is
    the same as WCharTy, and in C  it is an integer type compatible with
    the type in <stddef.h>.

This fixes PR15815.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181587 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/PrintfFormatString.cpp b/lib/Analysis/PrintfFormatString.cpp
index 8f151b9..60f9517 100644
--- a/lib/Analysis/PrintfFormatString.cpp
+++ b/lib/Analysis/PrintfFormatString.cpp
@@ -372,7 +372,7 @@
     case ConversionSpecifier::CArg:
       if (IsObjCLiteral)
         return ArgType(Ctx.UnsignedShortTy, "unichar");
-      return ArgType(Ctx.WCharTy, "wchar_t");
+      return ArgType(Ctx.WideCharTy, "wchar_t");
     case ConversionSpecifier::pArg:
       return ArgType::CPointerTy;
     case ConversionSpecifier::ObjCObjArg: